Unconditional
Trust or structural assumption

Non-colluding servers

In plain words

The assumption that the servers never compare notes. Each one alone learns nothing. Together they could.

Why it matters: Common in privacy systems run jointly by independent organizations.

The details

On the map: the servers never pool what they saw

The assumption that the servers in a protocol never pool what they saw. Each server alone learns nothing. Together they could reconstruct your query or data.

It is common in practice as "split trust" between independent organizations, for example Prio-style private aggregation[1], which was used for COVID exposure-notification analytics[2].

Rests on

If any of these breaks, this breaks too.

If it falls

If this breaks, these break with it.

References

  1. Henry Corrigan-Gibbs and Dan Boneh. Prio: Private, Robust, and Scalable Computation of Aggregate Statistics. NSDI 2017. usenix.org/conference/nsdi17/technical-sessions/presentation/corrigan-gibbs
  2. Internet Security Research Group. ISRG Prio Services for Preserving Privacy in COVID-19 Exposure Notification Apps. 2020. abetterinternet.org/post/prio-services-for-covid-en